Translating circuits



Mar. 13, 1923. 1,448,702

J. R. CARSON v TRANSLATING CIRCUITS Filed July lO, 1920 INVENTOR ATTORNEY Patented Mar.' is, 1923.'

PATENT OFFICE.

UNITED STATES,

JOHN R. CARSON, OF MONTCLAIR, NEW JERSEY, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH CO., A CORPORATION F NEW YORK.

TRANSLATING CIRCUITS.

Application led July 10, 1920. Serial No. 395;3OS.

To all whom it may concern.'

Be it known that I, JOHN R. CARSON, residin vat Montclair, in the county of Essex and tate of New Jersey, have invented certainv Improvements in Translating Circuits,

of which the following is a specification.

This invention relates to translating devices, and more particularly` to translating devices in the nature of modulators or detectors. f

One of the objects of this invention is to increase the output energy of a modulator, preferably of the vacuum tube type, al-

though the invention is equally applic-able to a vacuum tube detector. o y

Other and further objects of the invention will be clear from the following-description when read in connection with the accompanying drawing, Figures 1 and 42 of l Which are schematic diagrams indicating the 'electrical equivalents of certa/in types of vacuum tube circuits, while Fig. 3 is a dia gram ofa circuit arrangement of a vacuum tube modulator embodying` the principles of i this invention.

It is a well-known fact that if an electro- Inotive force E is impressed on the input circuit of a three-element vacuum tube arnplifier of amplification constant u and an external or load impedance z, its behavior as an amplifier may be correctly described by postulating an electromotive force aE impressed on vthe load impedance z through a resistance Rn, Where R., is the internal rei 'sistance of' the tube, and is given by the formulal 1 H1 R., @Eb (l) n 0 I being the plate current and Eb being the steady plate-filament potential difference. see'articles by H. J. van der Bijl'in the hysical Review of September, 1918, and in The Proceedings of the Institute of Radio 5 Engineers of April, 1919,-as well as an article by John R. Carson in -The Proceedmgs of the Institute of Radio Engineers of April,

'Iliislaw has led to the concept of the 0 equivalent circuit of the vacuum tube am plier, indicated schematlcally in Fig. l, and has been a very valuable aid 1n interpreting the behavior of amplifiers of this character and in designing the associated circuits. It may also be shown that a vacuum tube, when functioning as a modulator, may be represented by an equivalentcircuit of almost equal simplicity. The equivalent modulator circuit may be inferred at once from the following theorem:

If a Voltage E is impressed on the input terminals of a three-elementvacuum tube feeding into a load impedance z, the operation of the' device as a modulator may be correctly described by postulating an E.

MQR; 2) l i impressed on the load impedance .a through,

The equivalent circuit is shown schematically in Fig. 2.

.The law formulated by this theorem and the character of the approximations involved are easily deduced from the treatment of the problem developed in the above mentioned article by John R. Carson in The Proceedings of the Institute of Radio Engineers, and for full details of the solution reference may be had to said article. For the purposes of the present application it will be suflicient to merely outline the method of solution.

Let J representthe variable output current of a three-element Vacuum tube, e the variable electromotive force impressed on the input terminalsand u the variable potential difference across the load impedance a. -It follows at once that J is a function of the expression ue-i-o and this function may be expressed in a power series expansion as follows:

The significance of the series formulation of apparent later.

Direct addition by columns of equation (6) regard being had to the identities of equation (5), shows that equation (6) satisfies the requirements of equation (4), and is, therefore, a formal solution.

The coefficients in equation (4) will have a clear physical significance if we express P1 in terms of the first differential; thus l 0l Pfr (ai.) 7

Comparing this expression with the first term of equation (4:), it will be seen that by definition it is equal to Rl-r so that We may write 01 l Pffblfn. 8)

which is equation Writing in a similar manner expression P2 in terms of the second dnerentlal, we have In equations (7), (8) and (9) Eb is the voltage of the battery in the plate circuit of the tube, and M has already been defined as the modulation factor of the tube. It will be seen that, disregarding the sign which in this instance has no significance7 the value of M as expressed in equation once from equation (9).

Combining equations (5), (6), (8) and (9), we have:

(3) follows at Rolls lU3 ZROPZ (fue 'l' q)1)@2 Ropa (#5 l 'U03 Bearing in mind the assumed significance of the v series of equation (5), it follows at once that any equation, as, for example, the

nth equation of group (10), is simply that of the current Jn in a circuit of impedance RO-l-a in response to the voltage given by the right hand side of the equation. Now writing @lz-2J, and ogz-ZJQ (relations which follow at once from the above physical considerations), the first equation of group (l0) may be Written:

and the second equation of the group n'(10) may be written:

(ROJfaJFMG-ozte) 12) Equation l1 is simply the equation for the current J1 flowing in the circuit of impedance RO-f-a in response to the impressed electromotive force ue,`and therefore, identify- '.ing J1 with the amplification current, leads at once to the law of the equivalent amplifier circuit as expressed in the early part of this specification, and as illustrated in Fig. l; Similarly, equation (6) is simply the equation for the current J 2 flowing in a circuit of impedance R04-a in response to an impressed E. M. F.

' soning presented by John R. Carson in the Radio nstitute article already referred to will be sufficient to indicate t-o one skilled in the art how the law of the modulator formulated by the theorem set forth in the early part of this specification may be obtained. A simple problem will now be dealt with to illustrate the application of the concept of the equivalent modulator circuit in deducing certain principles which have a direct and important bearing on the design of the circuits of the vacuum tube modulator of this invention.

Let the impressed electromotive force E be the sum of two sinusoidal functions; thus,

E=E1 sin pit-tE, sin pet (13) In'this equation the first component of E may be regarded as the carrier wave and the second component as the signal wave to be impressed upon a modulator. y In substituting this expression for E in equation (2') to evaluate the fictitious electromotive force supposed to be acting in the equivalent Inodulator circuit of Fig. 2, it is essential that E be Written down as a complete real time MRO Where Z (p1) and Z (p2) are the absolute values of the impedance R04-2 to currents of fre uencies and 5;; respectively and function, since the operation of squaring is in effect a frequency transformation. By virtue of equation (13) We have:

61 and 62 are the correspondingphase angles. By aid of equation (14) formula (2) becomes, after simplification and "a rearrangement In the above expression -V :MRoEi :MROEZ 1 .ZUM HP2) Equation (15) represents the effective electromotive force acting in the equivalent and V2 EE 1 2 12 (Mmm) Zut) Zuw) (Zut -zw of frequency I of fre uenc Zampa q y respectively. The first factor represents -the electromotive force impressed upon sald 1mpetlance as expressed by the term MVIV2 2 in equation (15). Y i l Suppose now that by proper design of an ordinary modulating arrangement, the load impedance acts like a pure resistance R at the frequencies corresponding f to p1, p2, (271*p2) and (Pfl-PQ' It! is at Once. ap' parent, by substituting Ro--R for e in equations (16) and (17), that themodulated currents expressed by each of said equations will be proportional to v and the modulated energyVV1 delivered tov modulator circuit and the current corresponding is calculated by the usual methods. The modulated output current is evidently that corresponding to the term of equation (15) Which includes the factor VIVZ; it consists of the following tWo components:

thei load impedance Will consequently be proportional tov This expression is a maximum When and when this consideration is satisfied, be-

COYIleS e) a 5Ro Let us noW assume that, in accordance With the present invention, the modulating arrangement ybe so designed that the load impedance acts like a pure resistance R lat the frequencies correspondin to p1, (p1-p2), and (ppi-02),butactsl1ke a short clrcuit to currents of the signal frequency gj; .y This condition may be practically 'realized when the carrier frequency is sufficiently large as compared with the signal frequency.r In this case the term Z(p2) be- Hcomes R0 instead of Ro-l-R, as in the preceding case, and consequently the modulated currents are proportional to 3nd le modulated energy W2 is proporiona o This expression is a maximum when and when this condition is satisfied, becomes 3 4 il @a SRO A comparison of equation (20) and (23) shows that for the same applied electromotive forces the modulated energy outputs W1 and W2 are related by It follows at once that a proper design of the associated circuits to satisfy the condition laid down in the second case results in a gain of 57% in the available modulated energy, as compared with the' first arrangement. Another immediatededuction from formulas (16) and (17) is that, although we are concerned only in transmitting radio or high frequency modulated currents, it is nec essary to provide a low impedance path in the output circuit for the low frequency sig' naling currents otherwise a serious loss in the modulated energy results. y

As a consequence of these deductions, the modulating circuit may be arranged in' accordance With the present invention, as

f shown in Fig. 3, in'which M designates a Wave of carrier frequency may be impressed upon the grid 3, through a transformer 5, and the signalh frequency zr may be impressed upon the grid circuit through a'transformer 6. The load circuit correpass to currents of the signal frequency 2?? thereby making the impedance of the load circuit substantially zero for currents of signal frequency. A second path, including a condenser 11 and a transformer 12, is provided for Atransmitting currents correspondin to the frequencies p1, (p1-H02), and (p1-p2) to the out-going circuit 13.

It will be seen that from the theoretical considerations already given, an increase of approximately 57% in the output energy may be obtained by providing the circuit 9-10 forming a by-pass of very low or substantially zero impedance for currents of signal frequency, thereby obtaining a result of considerable importance in the art of modulation. It will be understood, of course, that where the signal wave is of variable frequency, the by-pass circuit should be tuned to the average signal frequency.

While the preceding discussion presupposes the use of a vacuum tube as a modulator,.the invention is equally applicable to the use of the tube as a detector, although in the latter case a slight functional modification of the arrangement is involved. Ordinarily, whenthe device is operating as a demodulator or detector, a low impedance path is provided for the signal frequency to be received. In accordance with this invention a by-pass would also be\Iprovided tuned to the carrier frequency so tl at the load circuit of the detector will have Substantially zero impedance) atl the carrier frequency. In short, regardless of whether thedevice is to be used as a modulator or detector, not only should a low impedancewpath be provided for l the frequencies which are'to be used,A as is now the practice, but a low impedancepath should also be provided for the frequencies not to be used. A

It will be obvious that the general vprinl ciples herein disclosed may be embodied in many otherv organizations widely different from those/illustrated, without departing quency upon the input circuit, andthe out L put'circuit being so arranged that itsimpedance Will be substantially zero for currents of the-signal frequency, but will act substantially'as apure resistance for frequencies in the neighborhood of the carrier frequency.

2. A modulating arrangement comprising a modulatlng device, means to impress a carrier frequency and a modulating signal freneighborhood ofthe carrier frequency.

4. A modulating arrangement comprising a vacuum tube modulator, means to impress a carrier frequency and a modulating signal frequency upon said modulator, and a load circuit for said modulator, said load circuit being so designed as to have substantially zero impedance for currents of signal frequenqy. A

5. modulating arrangement comprising a vacuum tube modulator, means to impress a carrier frequency and a modulating signal frequency upon said modulator, and a load Vcircuit for said modulator, said load circuit vbeing so designed as to have substantially zero impedance for currents of signal frequency, and to have an impedance Which acts substantially like a pure resistance for frequencies in the neighborhood of the carrier frequency. l

6. A modulating arrangement comprising a vacuum tube modulator having an input and an output circuit, means to impress a carrier frequency and a modulating signal frequency upon the input circuit, and aload circuit associated with the output circuit, said load circuit being so designed that its impedance Will .be4 substantially zero for currents of signal frequency.

7. A modulating arrangement comprising a-vacuum tube modulator having an inputand an output circuit, means to impress a carrier frequency and a modulating signal frequency upon the input circuit, and a load circuit associated with the output circuit, said load circuit being so designed that its impedance will be substantially zero for currents of signal frequency, and will act substantiallyA like a pure resistance for frequencies in the neighborhood of the carrier frequency.

. 8. A` modulating arrangement comprising a vacuum tube modulator having an input and an output circuit, means to impress a carrier frequency and a modulating signal frequency upon the input circuithan outgoing circuit'associated with the inputcircuit, and a by-pass bridged across the output circuit, the impedance of said by-pass being substantially zero for currents of signal frequency.

9. A translating arrangement comprising a device for translating a plurality of frequencies impressed thereon into combinati-ons of said frequencies, means to impress n a plurality of frequencies on said device for purposes of` translation, and a load circuitV associated. With said translating device, said load circuit being so designed that its impedance Will be substantially zero at one of said frequencies, but Will act substantially as a pure resistance for frequencies in the neighborhood of other frequencies impressed upon said translating device.

10. A translating arrangement comprising a translating device, means to impress a plurality of frequencies including a current of loW signaling frequency upon said device for purposesof translation, and a load circuit associated with said translating device, said load circuit being so designed that its impedance Will be substantially zero at said signaling frequency. i

l1. A translating arrangement comprising a translating device, meanstoimpress a p urality of frequencies including a current of low signaling frequency upon sald device for purposes of translation, and a load circuit associated With said -translating device, said load circuit being so designed that its impedance Will be vsubstantially zero at said signaling frequency but Will act substantially as a pure resistance for frequencies in the neighborhood o-f other frequencies impressed upon said translating device.

In testimony whereof, l have signed my name to this specification thisy 8th day of July 1920.

JOHN R. CARSON.

qi page 2, yliine 31, for`equ1tiyon V (Certcm of Correction.

111s herebycertified that in Letters-Patent No. 1,148,7o2',.gm1ed Merch 13, 1923, upon the application of JohnJRpCarson, of Montclair, New Jerse for an iml.provemont 1n Translating Circuits, errors appear -in the printe specification' requiring orrection as follows: Page; 1, line 375 for formula @1 e di- 61. R11-e323@ Ro-b Same page, line 75, for equation 1 01 1 l l pas? im) md *Pim-1,); ,1, page, line 40, for equation 1.2 pege,l1ine 46, for equation sind that the said Letters Patent should be read with these corrections therein that the' same may conform to the record of the case in the Patent Oieo.

y, Signed and'sea'led this' 14th dafyof August, A. D., 1923. v

[om] RL FENNING,

' Aetzng 00mm'rse'onea `of Patens, 

